1. Field of the Invention
The present invention relates to an anti-skid control system for use in an automotive vehicle for driving rear road wheels, and more particularly to an anti-skid control system for controlling braking force applied to front and rear road wheels in braking operation to prevent the road wheels from being locked.
2. Description of the Prior Art
In order to prevent road wheels from being locked, there has been employed an anti-skid control system which controls the braking force by decreasing, increasing, or holding a hydraulic braking pressure supplied to each wheel brake cylinder operatively connected to each road wheel.
According to the anti-skid control system, in general, detected is a rotational speed of each road wheel or wheel speed. On the basis of the maximum value of the wheel speeds of the four road wheels, an estimated vehicle speed is calculated, and the hydraulic braking pressure in each wheel brake cylinder is controlled in response to a difference between each wheel speed and the estimated vehicle speed, so as to obtain the maximum coefficient of friction. In the anti-skid control system, as disclosed in Japanese Patent Laid-open publication No. 62-155160 for example, it has been so arranged that the hydraulic braking pressure in the wheel cylinder is increased with intervals, i.e., a pulse increasing mode signal is output, when a high peak of the wheel speed is caused.
In the anti-skid control system for use in a vehicle for driving rear road wheels, generally it is so arranged that so called independent control mode is employed for each of the front road wheels and so called low-select simultaneous control mode is employed for the rear road wheels. According to the low-select simultaneous control mode, a braking force, which is to be applied to one of the rear road wheels rotating at the lower speed and having a tendency to be locked first, is applied to both of the right and left rear road wheels simultaneously. Consequently, in the case where the vehicle having that anti-skid control system is running on a road of a low coefficient of friction, if the anti-skid control starts with a shift gear engaged when the vehicle is in so called engine-braking condition, the wheel speeds of the rear road wheels will be hardly recovered by reducing the hydraulic braking pressure in the wheel brake cylinders of the rear road wheels. Since the estimated vehicle speed used for the above-described anti-skid control system is calculated on the basis of the maximum wheel speed of the four road wheels, if the right and left front road wheels are also rotated synchronously in the above condition, wheel speeds of both front road wheels will be reduced. Consequently, the estimated vehicle speed is set to be low, so that all the wheel speeds of the four road wheels will be reduced to cause the road wheels to be locked rapidly.
FIG. 7 shows the anti-skid control operation of the above-described prior art. When pulse mode signals are output at a position (A) simultaneously with respect to the right and left front road wheels FR, FL, both wheels FR, FL rotate synchronously, so that the wheel speed Vwf of the wheels FR, FL vary as indicated by a solid line in FIG. 7, wherein V indicates an actual vehicle speed. Therefore, as indicated by a broken line, the estimated vehicle speed Vs is low at the position (A) as in a state at the time of initiation of the anti-skid control. Consequently, the wheel speed Vwr (as indicated by two-dotted chain line) of the rear road wheels RR, RL will be reduced less than the estimated vehicle speed Vs, so that the road wheels RR, RL will be locked at a position (B). Then, the road wheels FR, FL also will be locked.